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From diatoms to killer whales: impacts of pink salmon on North Pacific ecosystems

Gregory T. Ruggerone*, Alan M. Springer, Gus B. van Vliet, Brendan Connors, James R. Irvine, Leon D. Shaul, Matthew R. Sloat, William I. Atlas

*Corresponding author:

ABSTRACT: In response to a climate regime shift in 1977 and general heating of the North Pacific Ocean, pink salmon Oncorhynchus gorbuscha abundance reached record highs during 2005-2021, comprising 70% of all Pacific salmon. Pink salmon are approximately 25 times more numerous in odd- than even-numbered calendar years in some major North Pacific ecosystems, a unique demographic pattern analogous to repeating whole ecosystem treatment-control experiments. We found compelling examples indicating that in odd years pink salmon predation can initiate pelagic trophic cascades by reducing herbivorous zooplankton abundance sufficiently that phytoplankton densities increase, with opposite patterns in even years. Widespread interspecific exploitation competition for common-pool prey resources can be dominated by pink salmon, as indicated by numerous biennial patterns in the diet, growth, survival, abundance, maturation, distribution, and/or phenology of ecologically, culturally and economically important forage fishes, squid, Pacific salmon and steelhead trout Oncorhynchus spp., seabirds, humpback whales Megaptera novaeangliae, and endangered southern resident killer whales Orcinus orca. In aggregate, the evidence indicates that open ocean marine carrying capacity in the northern North Pacific Ocean and Bering Sea can be mediated by top-down forcing by pink salmon and by ocean heating, and that large-scale hatchery production (~40% of the total adult and immature salmon biomass) likely has unintended consequences for wild salmon, including Chinook salmon O. tshawytscha, and many other marine species. Further investigation of pink salmon's effects on other species will enlighten our knowledge of ecosystem function and the important role top-down forcing plays in the open ocean.